When a vehicle is at rest or moving at a lower velocity, a large force is required to rotate the steering wheel for veering the tires. Especially, so-called FF (front-engine, front wheel) vehicles which have become increasingly popular in recent years require still greater force to be applied to the steering wheel, because the front tires of this kind of vehicle bear greater weight.
It is known that a power steering system augments the torque that is applied to the steering wheel of a vehicle by the driver. This steering system produces a driving force in response to the manual steering force exerted by the driver, and the produced force is transmitted to the steering mechanism. Most power steering systems which are now put into practical use are hydraulic in structure. In particular, such a hydraulic system includes a control valve, a hydraulic cylinder, etc., and produces an auxiliary steering force by moving oil in response to the steering force applied by the driver.
Unfortunately, the aforementioned control valve, oil cylinder, and so forth are bulky. Further, pipings for interconnecting these components cannot be bent with a curvature less than a given value to prevent occurrence of a large pressure loss. In addition, a hydraulic system requires seals to be certainly installed for preventing oil leakage. Furthermore, it is cumbersome to install such a hydraulic system. For these reasons, it is difficult to install a power steering system in a vehicle having a small space available for the installation such as an FF vehicle.
Meanwhile, a larger force is needed to steer a vehicle as the velocity decreases, and vice versa. The prior art power steering system operates at all times, irrespective of the vehicle velocity. Therefore, no problem arises at lower velocities, but the force necessary to steer the vehicle is inordinately reduced at higher velocities. This may introduce the possibility that a driver who is unaccustomed to the power steering system rotates the steering wheel through an excessive angle, thus incurring a danger. Also, that a somewhat large force is required to steer the vehicle when it runs at a high velocity makes it easier for drivers accustomed to power steering to drive the vehicle. Thus, a power steering system has been proposed which measures the velocity of the vehicle and produces an auxiliary torque matched to the velocity. However, the control system of the steering system is very complicated and hence it is expensive.
In view of the foregoing situations, the present applicant has proposed a motor-driven power steering system in Japanese Patent Application No. 174753/1982, in which when the velocity of the vehicle is lower than a predetermined value an electric motor is energized according to a detected torque to amplify the torque applied to the steering wheel by the driver, while when the value is exceeded the motor is de-energized. This proposed system provides a relatively simple structure, and it functions satisfactorily as a power steering system at low velocities, where an auxiliary torque is necessitated.
Conventional velocity-responsive type power steering systems including the system described just above are inactive at high velocities. If the force needed to steer the vehicle at high velocities is greater than that needed at moderate velocities, then the driving would be facilitated.